Posted: 4/9/2005 7:20:18 PM EDT
| How can we be sure that giant stars are really larger than main sequence stars? |
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I'm not an astronomer, and they may have a better way, but here's how I'd do it. First we have the discreet elemental spectra for elements from measurements made here on earth. Using this information we can determine the red shift of a given star. Knowing the universe is expanding and the rate of that expansion, we can use this information to determine distance based on the velocity given by the shift. Then you can measure the width of the star from the visual observation and factor in the distance to determine the diameter. That should work. But as I said, the people that do this for a living probably have an even better way. |
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You wouldn't really use "width", since I don't think we have any equipment that can resolve even something the size of a star at any appreciable distance. They are, for all intents and purposes, "point sources". You typically use corrected spectra/composition, and luminosity to place the star in its proper spot on the HR diagram. The HR diagram is a plot of absolute magnitude (brightness) versus a particular filter function (B-V). (I have only taken one formal astrophysics class.) Jim |
How do you go about determining physical size? I suppose you could do something mathematically using physics theory with mass vs brightness data to determine the density of the star and then the volume it occupies in space. It seems like we should have some way of doing a direct measurement of size without relying on magnitude. |
